Variable,nonlinear impedance

ABSTRACT

Use of an electronic circuit constituting a variable, nonlinear impedance having a bridge with four branches, each comprising a diode, two of the diagonal points of the said bridge being fed by an adjustable current and the other two diagonal points of the bridge constituting the terminals of the impedance, the direction of conduction of the diodes being the same at the diagonal points fed by the adjustable current in order to compensate for nonlinearity and/or regulate the gain of an electronic device.

United States Patent Kudelski 14 1 Aug. 1, 1972 54] VARIABLE, NONLINEAR IMPEDANCE 3,428,762 2/1969 0661166 81 al ..179/1002 K Inventor: Stefan d l i Le Munbsup Erath et al K lausanne, Chemin De La Croix, OTHER PUBLICATIONS Switzerland lBM T h a1 1) 1 B 11 b v w kl ec me we osure 11 em y an in e [221 1969 V01. 2 N0. 4 Dec. 1959 page 29 [21] A l, N 887,301 Millman & Taub McGraw-l-lill Electrical & Electronic 2 Engineering series 1956 (p. 445, 447) Title Pulse and Digital Circuits [30] Foreign Applicafion Pnomy 1 Korn & Korn Electronic Analog & Hybrid Compu- Dec. 26, 1968 Switzerland ..19290/68 ters G i 1 (p- 216) [52] [1.8. CI. ..2307/321, 307/310, 323/75 F Primary Examiner-Donald D. Forrer [51] Int. Cl ..H03k 3/26 Assistant Examiner-R. E. Hart [58] Field of Search ..307/321, 310, 255, 257; A torney-James M. Heilman and l-leilman & Heilman [57] ABSTRACT [56] References and Use of an electronic circuit constituting a variable, UNITED STATES PATENTS nonlinear impedance having a bridge with four branches, each comprising a diode, two of the 3,115,603 12/1963 Fluegel ..323/75 F diagonal points of the said bridge being fed by an 3,122,654 2/1964 Roalef ..328/208 jumble current and the other two diagonal points of 3,166,276 1/1965 Goerner et al ..323/9 the bridge constituting the terminals of the 3,177,427 4/1965 Kuntz et a1. ..323/75 F pedance, the direction of conduction of the diodes 3,287,567 11/1966 066 12,... ..307/321 being the same at the diagonal points fed by the ad- 3,526,791 9/ 1970 Cod1ch1n1 ..307/310 justable current in order to compensate for nonlineari 3,222,547 12/1965 B031] 611 al ..307/255 ty and/or regulate the gain of an electronic device 3,471,715 10/1969 Castelll ..307/255 3,031,588 4/1962 Hilsenrath ..307/255 4 Claims. 8 Dmwim Finnrac PATENTEDAUS 1 I972 sum 1 or 5 Idc Fig.2

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INVENTOR STEFAN KUDELSKI SHEET S [If 5 INVENTOR STEFAN KUDEL I VARIABLE, NONLINEAR IMPEDANCE I This invention relates generally to a variable impedance device, and more particularly, to a nonlinear variable impedance device.

The object of the present invention is the use of an electronic circuit constituting a variable, nonlinear impedance which has a bridge with four branches, each- FIGURES FIG. 1 is a schematic diagram of the present invention.

FIG. 2 is an explanatory curve therefor;

FIG. 3 is a schematic diagram of a recording device with distortion compensation and variable gain.

FIG. 4 is a schematic circuit of two symmetrical transistors for regulation within a wide range of gain.

FIG. 5 is a simplified schematic diagram. FIG. 6 is a detailed schematic diagram of FIG. 3.

FIG. 7 is a schematic diagram of an application of the present invention.

FIG. 8 is a schematic diagram of a circuit which may be used with the schematic diagram of FIG. 7.

DETAILED DESCRIPTION The circuit shown in FIG. 1 comprises a bridge 1 having four branches, each having a diode. The bridge- 1 comprises four diagonal points A, B and C, D. The points A and B are connected to a source of alternating current 3, while the points C and D are fed by a selected direct current I This circuit, between the points A and B, has a symmetrical, nonlinear impedance the curve of which is shown in FIG. 2. It can be seen that as the alternate voltage E between A and B increases, the impedance also increases, becoming infinite at the moment when the alternate current I equals the direct current I passing through the bridge, provided that the feed device of the bridge also has an infinite AC impedance.

It is known that electronic devices such as amplifiers, oscillators or sound recorders, and in particular the assembly of the latter consisting of the magnetic recording head and magnetic tape, have an impedance or a gain which decreases as soon as the voltage, that is to say, the signal, increases beyond certain limits,v which corresponds to a characteristic which is complementary to that shown in FIG. 2. By combining the circuit described with any of the aforementioned electronic devices by means of a member which suitably adapts the amplitude of the signals, one can therefore compensate for the deformations or distortions of the one by the deformations of the other and therefore decrease the overall distortion of the assembly. Furthermore, as the impedance of the bridge is inversely proportional to the direct current which passes through it, it is possible to regulate the gain of the assembly by modifying the said current. It should be noted that for a given type of diode, the curve of the voltage over the ratio of the signal current to the direct current of the bridge is constant for a wide range of currents. This curve is also substantially constant as a function of the temperature Advantageous use may be made in the case of the recording circuit of a magnetic recorder. The fact that the gain can be varied makes it possible to obtain a limiting device by means of which one can avoid the saturation of the magnetic tape without excessive distortion in the event that the input signal should happen to exceed the normal maximum value. At the same time, the complementary nature of the characteristics of the diode bridge and the magnetic tape make it possible to reduce the distortion of the recorded signal by one order of magnitude.

Another contemplated use is for an oscillator. A sinusoidal oscillator is essentially a selective amplifier connected on itself. At the frequency at which the loop gain is greater than 1 (phase shift 0), the amplifier starts to oscillate. This oscillation increases until a regulating element decreases the gain andstabilizes the amplitude thereof. Such a regulation has been effected by the saturation of an element, but the signal obtained was deformed by this saturation. Use has also been made of an element heated by the signal and the impedance of which varies with the temperature so as to reduce the gain. This manner of procedure entails, however, drawbacks such as the slowness of the response and the dependence of the output signal on the room temperature. By inserting the diode bridge described into the circuit of the oscillator and causing the direct current which passes through it to depend on the amplitude of the signal produced, the oscillator is stabilized without difficulty with optimum time constants. Furthermore, as the base amplifier produces a certain distortion which is complementary to that of the diode bridge, the AC amplitude can be suitably selected on the bridge so as to decrease this distortion by one order of magnitude. In the case of a high quality oscillator, a second diode bridge can be added in a feedback chain of the amplifier in order to perfect the correction.

It may be advantageous to feed one of the points A or B of the bridge by an auxiliary current in order to compensate for the differences in characteristics of the diodes or in order to create a distortion of even order, for instance to compensate for the distortion produced at another point of the assembly.

FIG. 3 is an example of an embodiment of a recording device with distortion compensation and variable gain. This circuit, which is a circuit with complementary transistors and 102, comprises a diode bridge 10, variable impedance means 11 for compensation of distortion by even harmonics, a resistor 12 for compensating for differences in the thresholds of the PNP and NPN transistors, an amplifier 13 acted on by voltage a series input resistor 18, and recording head 14 and a bias magnetization generator 15. The diagram also shows the input terminals 16 and the terminals 17 for applying the gain control voltage to the base of a control transistor 103.

FIG. 4 shows an assembly with two symmetrical transistors T, and T for the regulation of sensitivity within a wide range of gain. There is concerned here another form of feed of the diode bridge which permits a variation of gain within a wide range. If one desires to I lating circuit.

vary the gain by means of the diode bridge rapidly, it is very important to feed it perfectly symmetrically, that is to say, the current I, (FIG. 5) must be as identical as greater, the assembly of FIG. 4 is preferable. In'this assembly, the sum of the currents of the transistors T, and T, is constant since they are both fed by a common source of constant current I. Thus, any increase in current of the transistor T, which supplies the current I, to

the diode bridge is accompanied by a decrease in current of the transistor T, of identical absolute value. As the device X (FIG. 4) supplies a constant current, the result is that I, will be identical to I, in absolute value.

FIG. 7 shows a part of an automatic sensitivity regu- In this circuit, 21 is the diode bridge, 22 the positive source of potential, 23 the input of the signal (values of between 4 p. amp and 400 1. amp effective), 24 the output of the signal, 25 the terminal for the application of the control voltage (values of between 0 and 4 V), 26 the common terminal, 27 and 28 the transistors T, and T, respectively and 29 a logarithmic circuit. The diagram also showsthe currents I, and I, the current I the direct current I,,, controlled to establish the operating point and the constant alternating current 1, It should be noted that the current gain of the transistors T, and T, must be very great in order to prevent the base current from disturbing the equilibrium. If the transistors available are of insufiicient gain, they are assisted by means of the amplifier circuit of FIG. 8.

One of the advantages of the use in accordance with the invention and its applications resides in the stability obtained with respect to variations in temperature, which stability permits the suppression of thermal compensation. As a matter of fact, the system has a nonlinearity which is very constant as a function of the temperature, and it is easy to determine the amount of this nonlinearity by the determination of the amount of current in the bridge, which makes it possible to vary it as a function of the temperature in order to adopt it to the required conditions. Furthermore, there is a very slight passage of control signal in the useful signal, and it is not necessary to use precision elements or elements of high stability.

1 From the above description it is obvious that the diode bridge can be used in a number of circuits to (1) limit the current in a non-distorting manner, (2) to' correct for distortion produced in many communication circuits, and (3) to control the current in a signal transmission circuit. The preferred form of the bridge circuit is shown in FIG. 4 where the bridge controls the input voltage applied to an amplifier'by presenting a shunt circuit having a variable impedance. The variable impedance controls the input current and varies the potential drop across an input resistor 18, thereby also controlling the applied voltage.

The embodiments of the invention in which an exclu- A sive property or privilege is claimed are defined as follows.

Wlilat is claime i l. n a contro a le nonlinear impedance connected to a pair of input terminals in series with a dropping impedance for conditioning the AC input voltage applied to a recording means, the improvement which comprises:

a. a coupling means connected between one end of said dropping impedance and said recording means;

b. a compensating circuit for correcting the waveform distortion in a recording means, said 1 compensating circuit connected across said recording means and including a first series resistor, a four-armed bridge, and a second series resistor; said four-armed bridge including a rectifier diode in each arm for continuous passage of current having a nonlinear amplitude responsive to its applied voltage;

. a direct current power source connected acro said compensating circuit for producing a direct current in all four bridge arms;

d. a first and second variable impedance connected i respectively across the first and second series resistance for varying the current through the bridge, said variable impedances each including the emitter-collector circuit of a transistor; and

e. control means connected to the bases of said transistors for varying the values of said variable impedances.

2. A controllable impedance as claimed in claim 1 wherein said control means for varying the variable impedance includes an adjustable direct current voltage applied to the bases of said transistors.

3. A controllable impedance as claimed in claim 1 wherein the four rectifier diodes in said bridge are poled so that each passes a continuous current supplied by said direct current power source.

4. A controllable impedance as claimed in claim 1 wherein the base electrodes of said transistors are respectively connected to the collector and emitter of third transistor.

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1. In a controllable nonlinear impedance connected to a pair of input terminals in series with a dropping impedance for conditioning the AC input voltage applied to a recording means, the improvement which comprises: a. a coupling means connected between one end of said dropping impedance and said recording means; b. a compensating circuit for correcting the waveform distortion in a recording means, said compensating circuit connected across said recording means and including a first series resistor, a four-armed bridge, and a second series resistor; said four-armed bridge including a rectifier diode in each arm for continuous passage of current having a nonlinear amplitude responsive to its applied voltage; c. a direct current power source connected across said compensating circuit for producing a direct current in all four bridge arms; d. a first and second variable impedance connected respectively across the first and second series resistance for varying the current through the bridge, said variable impedances each including the emitter-collector circuit of a transistor; and e. control means connected to the bases of said transistors for varying the values of said variable impedances.
 2. A controllable impedance as claimed in claim 1 wherein said control means for varying the variable impedance includes an adjustable direct current voltage applied to the bases of said transistors.
 3. A controllable impedance as claimed in claim 1 wherein the four rectifier diodes in said bridge are poled so that each passes a continuous current supplied by said direct current power source.
 4. A controllable impedance as claimed in claim 1 wherein the base electrodes of said transistors are respectively connected to the collector and emitter of a third transistor. 